1. Field of the Invention
The present invention generally relates to an optical transmitter. In particular, the present invention relates to a wavelength locker is provided in the optical transmitter for locking the wavelength of an optical signal.
2. Description of the Related Art
The wavelength-division-multiplexing (WDM) technology has been a great area of research as it can allow for the transmission of increasing information without largely changing the backbone network of an optical communication system. Consequently, the possible employment of both the 100-GHz spacing wavelength-division-multiplexing technology which is currently used in the existing system, as well as the 50-GHz dense wavelength-division-multiplexing (DWDM) technology has been of great interest. In order to implement such a dense wavelength-division-multiplexing technology, it is necessary for an oscillating wavelength to be maintained within an error range not to exceed 20 pm. Because it is difficult to implement the dense wavelength-division-multiplexing technology merely by securing the stability of an oscillator itself, optical transmitters are widely used and are individually provided with a wavelength locker disposed outside the oscillator. Such wavelength lockers are classified as either an exterior type which is provided outside of optical transmitters, or an interior type which is integrated inside of optical transmitters. In view of easy system construction, price and other factors, it is expected that the demand for the interior type wavelength lockers will increase.
FIG. 1 schematically illustrates an optical transmitter provided with a wavelength locker of the prior art. The optical transmitter comprises a laser diode (LED) 110, a Fabry-Perot filter 120, a photodiode (PD) 130, an analog/digital converter (ADC) 140, a controller 150, a digital/analog converter 160, a bias circuit 170, and a thermoelectric cooler (TEC) 180.
The laser diode 110 is a semiconductor component, which projects light with a predetermined wavelength through its front and rear faces. The light projected through the rear face is used for monitoring the oscillating wavelength of the laser diode 110. That is, the stabilized optical transmission can be executed by monitoring the oscillating wavelength of the laser diode 110 and correcting a wavelength error when the error occurs. The wavelength of the laser diode 110 changes as the operating temperature of the laser diode 110 changes.
The Fabry-Perot filter 120 may consist of a spacer formed from a linear optical material and two reflecting layers each being formed on one of the front and rear faces of the spacer, wherein the Fabry-Perot filter 120 has a predetermined transmittance spectrum depending on the thickness and refractive index of the spacer. That is, if the light projected through the rear face of the laser diode 110 is incident to the Fabry-Perot filter 120, the power of the transmitted light will vary depending on the wavelength of the light. Therefore, it is possible to find the wavelength of the light by measuring the power of the light projected from the Fabry-Perot filter 120.
The photodiode 130 converts the light that is incident to the Fabry-Perot filter 120 into an electrical signal and outputs the electrical signal.
The analog/digital converter 140 converts the electrical signal into a digital signal and outputs the digital signal to the controller 150.
The controller 150 perceives the power of the light from the electrical signal and finds the wavelength of the light based upon the power of the light. If the wavelength determined from this operation does not match a predetermined wavelength value, then the controller 150 outputs a control signal for correcting the wavelength error.
The digital/analog converter 160 converts the control signal into an analog signal and outputs the analog signal to the bias circuit 170.
The bias circuit 170 applies electric current to the thermoelectric cooler 180 in response to the control signal output by the controller 150. That is, the level of the applied electric current is determined by the control signal.
The thermoelectric cooler 180 is operated by the electric current applied by the bias circuit 170 and functions to maintain the temperature of the laser diode 110 to be constant at a predetermined temperature.
FIG. 2 is a flowchart showing the setting procedure of the optical transmitter shown in FIG 1. FIG. 3 is a drawing illustrating the setting procedure shown in FIG 2. The setting procedure consists of a channel-setting step 210, a working temperature-setting step 220, and a Fabry-Perot filter-aligning step 230.
The channel-setting step 210 is a step for setting a specific channel from a group of usable International Telecommunication Union (ITU) channels. Referring to FIGS. 3a and 3b, one channel 310 is selected from a series of ITU channels and the oscillating wavelength of the laser diode 110 is biased to an edge of the set channel 310 prior to the working temperature-setting step 220.
The working temperature-setting step 220 adjusts the working temperature of the laser diode 110 in order to match the oscillating wavelength to the center of the selected channel 310. According to this step, it can be seen that the oscillating wavelength moves from the edge to the center of the set channel 310 (320→350).
The Fabry-Perot filter-aligning step 230 is a step for aligning the oscillating wavelength of the laser diode 110 and the transmittance of the Fabry-Perot filter 120. Referring to FIG. 3c, the oscillating wavelength of the laser diode 110 is laid in the flat region of the transmittance spectrum prior to the Fabry-Perot filter-aligning step 230. For this reason, a problem arises because it is impossible to find such a wavelength change, and because the power of the light projected from the Fabry-Perot filter 120 changes slightly even if the oscillating wavelength of the laser diode 110 fluctuates within the flat region. Therefore, the Fabry-Perot filter 120 is aligned (340→350) in such a manner that the oscillating wavelength can be fixed in a wavelength region where the change of the transmittance of the Fabry-Perot filter 120 is large—i.e. close to a wavelength that exhibits a mean transmittance of the maximum and minimum transmittances. At this time, the aligning step 230 is conducted by aligning the incident angle θ of the light that is incident in the Fabry-Perot filter 120 and the optical axis of the Fabry-Perot filter 120. That is, the aligning step 230 is conducted by aligning the optical axis of the laser diode 110 and the optical axis of the Fabry-Perot filter 120. It can be seen that the oscillating wavelength of the laser diode 110 is fixed close to the wavelength which exhibits the mean transmittance of the maximum and minimum transmittances of the Fabry-Perot filter 120 while passing through the aligning step 230.
FIG. 4 shows a transmittance spectrum for the Fabry-Perot filter 120 shown in FIG. 1 in relation to the incident angle θ. The transmittance spectrum is obtained when the thickness of the spacer is 20 μm and the coefficient of finess is 10. From the drawing, it can be seen that the transmittance spectrum rapidly moves to the shorter wavelength side (410→420→430) as the incident angle θ increases by increments of 1° from 0°. That is, it can be seen that the wavelength moves 0.3 nm every time the incident angle θ changes 1°, and thus the aligning error of the Fabry-Perot filter 120 should be limited to approximately 0.1° considering that a 50-GHz ITU channel grid is 0.4 nm. However, there are problems in that it is extremely difficult to conduct such an alignment manually, and the separate use of an expensive active alignment installation will excessively increase production costs.
As described above, wavelength lockers for optical transmitters of the prior art experience problems with the alignment of the Fabry-Perot filter. Either manual alignment is required which is an extremely difficult procedure considering the precision required, or an active alignment installation must be used which is a very expensive procedure increasing the production cost of the optical transmitter.
Accordingly, there is a need to provide a wavelength locker for an optical transmitter that does not require the fine geometrical alignment of the Fabry-Perot filter.